Present plotters employ an ink delivery system comprising an ink-jet pen fed by an ink-loaded foam reservoir. A vent hole is formed near the top for providing atmospheric pressure in the foam region so that the ink can be drained. A filter is situated in the bottom of the reservoir to trap particles. A manifold attached to the bottom of the reservoir distributes ink from the reservoir to an ink-jet print head, which comprises a plurality of nozzles in a nozzle plate. The nozzles are associated with thermal means, such as thin film resistors, to form selected bubbles of ink, which are propelled to a printing medium, such as paper. However, the foam ink delivery system has several drawbacks.
For example, air bubbles can be trapped in the foam and create air channels, resulting in depriming of the ink-jet print head and the nozzles. Also, it is difficult to completely fill the foam with ink during manufacturing. Additionally, a very large filter is required at the bottom area of the reservoir or pen body to permit an even and full drainage of the foam. The high cost of such filters increases the cost of the system considerably.
Further disadvantages include the possibility of migration of air bubbles down the manifold, below the filter area, resulting in "unprimable" bubbles. Also, the foam is about 70% efficient; that is, only about 70% of the ink in the reservoir can be conveyed to the print head.
In addition, the ink is unstable if the pen is moved onto its side. Due to the instability, shipping of the pen often results in depriming, thereby necessitating a more complex preparation and again increasing the cost of the pen.
The vent hole allows air-ink contact, which may affect the long-term viscosity of the ink. Also, ink drainage from one batch of foam to the next is not repeatable. Thus, the operator of the plotter cannot tell whether a pen can complete a plot.
Finally, ink level in the reservoir is detectable only by one of two methods. In the first method, nozzle firings are counted. However, it is easy to lose count of the number when the pen is moved to a service station on the plotter if, for instance, the user wants a different color pen. The plotter then loses count for the old pen firings.
Alternately, a tachometer method may be employed. This method consists of a mechanically acutated memory that is located on the pen after a plot, the drop count is converted to the volume of ink used up, and the memory on the pen changed accordingly. Before a new plot, this memory is checked to determine if enough ink is present to start plotting.
Thus, it is clear that the present foam system has several drawbacks which detract from its usefulness. Accordingly, an improved ink delivery system is needed which avoids most, if not all, the foregoing problems.